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I'm trying to calculate the habitable zone for different planets based on albedo and greenhouse effect potency. I'm assuming that a habitable planet has an albedo of 20 - 40 and a greenhouse potency of 0.5 - 5 (Earth = 1, Mars = 0, Venus = 200).

What is the habitable range for surface temperatures? I know that oceans begin to boil off at about 40 - 50 °C, as that is what will most likely happen on Earth in 1 Billion years, and that oceans can exist at temperatures slightly below 0 °C if they are very saline.

The question is where exactly do we draw the line? Is the maximum temperature 25 °C, 30 °C (what I'm using currently), 35 °C, 40 °C, or something else? And is the minimum temperature 0 °C, -5 °C (which is what I'm again using currently), -10 °C, or again something else?

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  • $\begingroup$ Between freezing and boiling. $\endgroup$
    – ErikHall
    Jan 1 at 19:16
  • $\begingroup$ @ErikHall Both of those change based on the conditions which means that isn’t very helpful. $\endgroup$
    – Joe W
    Jan 1 at 19:38
  • $\begingroup$ We have one and only one data point - Earth - and it's a trivial Google search to discover that data point. How that data point relates to albedo and greenhouse potency is likely a relationship that's never been investigated and, since there's only one data point, is anybody's guess. In short, you're looking for mathematical certainty for a science-fiction quesiton. Also see [1] and [2]. $\endgroup$
    – JBH
    Jan 1 at 21:55
  • $\begingroup$ "Oceans begin to boil off at 40-50* C". There's always water vapor rising from liquid water, just varying rates according to temperature and pressure. wiredchemist.com/chemistry/data/vapor-pressure $\endgroup$ Jan 5 at 1:51

3 Answers 3

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At temperatures above around 200 Celcius, water vapour is energetic enough to leave the atmosphere of an earth-like planet, making oceans of any type impossible. [1]

For a more realistic limit, NASA estimates that certain organisms around geothermal vents 3.5 billion years ago lived in 85 degree conditions, while the ambient ocean temperatures were around 75 degrees [2]. Given that earth had previously spent a good time being much hotter than this, and given that the NASA analysts got this data by reconstructing the oldest enzymes they could based on reverse engineering current organisms, I think it's safe to say that 75-85 degrees is the absolute upper limit. Any higher, and the first simplistic forms of life would be unable to survive the temperatures, meaning the planet would be totally sterile.

For the low end: While salt is one way of depressing freezing temperature, there is one other naturally occurring, relatively non-toxic option, which happens to work even better: Magnesium Chloride (MgCl2). In non-lab conditions, it can reduce the freezing temp of water to about -15 degrees[3]. It is not toxic to humans but high levels of it can lead to chloride concentration issues in plants, which can be deadly to the plant. As such, it would be wise to mix salt and MgCl2, so as to get most of the benefit with less danger for toxicity. Native plants would likely evolve to cope with the low levels of chloride and it would not be a problem.

As for life surviving at -15 degrees: The only real limiting factor on cells surviving lower temps is that water freezes and either outright kills the organism or puts it in cryostasis, so as long as there is a liquid medium, simplistic life can survive. [4]

In summary, the limit is between -15 and 85 degrees celcius. I don't see why life would have any trouble emerging in simplistic form at these temperatures, and I doubt anyone knows whether it's possible for complex life to evolve at these temperatures, so do as you please with levels of complexity.

Hope that helps!

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  • $\begingroup$ Sorry, realised this answer is a bit long. Shortened version: Between -15 / 85 celcius for habitability, and if you want to go below zero you should use a mixture of salt and magnesium chloride. Anything below -15 and no liquid oceans (or life), anything above 85 and you COULD have liquid oceans but there would be no life. Best of luck!! $\endgroup$
    – Valitenci
    Jan 5 at 7:51
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I know that oceans begin to boil off at about 40 - 50 °C

I wouldn't take that too seriously.

For example, if a planet has a dark side and a light side, and hot water vapor precipitates when it reaches the dark side only to flow to the light side and warm up again, an equilibrium could be reached. And, even if the planet loses some water vapor, occasional comet impacts could replenish the water that is lost.

Likewise, an ocean doesn't have to be pure water. Mix various substances with water and the boiling and freezing temperatures can change, as you note in passing.

If you mix your water with an anti-freeze agent that local flora and fauna have evolved to tolerate (e.g., sea salt water freezes at about -17.8°C and commercial antifreeze agents for cars can prevent freezing prior to about -37°C), oceans can get much colder than 0°C. 80 proof vodka, which is roughly 40% ethanol and roughly 60% water, won't freeze in your kitchen freezer, it has a freezing temperature of -26.95°C, and 100 proof vodka (which is 50% alcohol) has an even lower freezing temperature of -40.43°C.

Also, who says that your oceans need to be made predominantly of water at all. Ethanol (i.e. the kind of alcohol found in alcoholic beverages) freezes -114.1°C sea level atmospheric pressures.

Other mixtures might have higher evaporation temperatures (which are also impacted by humidity and air pressure) and boiling temperatures.

There is no reason to be terribly exact, because many biological adaptation can expand the habitable zone (think of the life in the near boiling water of thermal sea vents), and chemistry, pressure, non-homogeneous conditions on a planet, differences in gravity, and differences in light exposure can impact the behavior of fluids.

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  • $\begingroup$ I had this idea too, but I personally slightly prefer ammonia since ammonia is a lot more common (because it only consists of two different elements [H, N]) than ethanol (which consists of three different elements [H, C, O]). $\endgroup$ Jan 23 at 14:02
  • $\begingroup$ @SussusAmogus Oceans of ammonia could work. $\endgroup$
    – ohwilleke
    Jan 23 at 14:12
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There is not really a hard limit for brief periods. However, oceans become less and less maintainable the further from Earth like conditions we get and will not be present after billions of years in some cases.

  1. Regardless of temperature, a humid atmosphere can only radiate 385 W/m^2 of IR into space. The reasons for this we complicated but have to do with the greenhouse effect of water causing more water to evaporate and leading to runaway escalating greenhouse effect in a geologically short period. This doesn't impose any limit on surface temperature but it imposes a pretty strict limit on effective blackbody temperature and defines the inner limit of the traditional habitable zone concept. This is pretty accurate for any planet with large Earthlike oceans. There might be a little more than that which can be safely scattered by clouds, ice, or reflective terrain, but not much. 385 W/m^2 translates to a blackbody temperature of 287.1 Kelvin, or 13.9 °C, which is pretty comparable to Earthlike conditions.

However, Earth is not in a runaway greenhouse state because the insolation at Earth's distance is just 340 W/m^2 and Earth's greenhouse effect is the reason it is able to maintain its moderate temperature.

  1. Moist greenhouse state. When the global average temperature breaks about 47 °C, a lot of water will get into the upper atmosphere and the result will be loss of hydrogen into interplanetary space unless the planet is much larger than Earth. This will usually further warm the planet and temperatures in the 60 or 70 °C range are viable. This usually happens before the runaway state, and can lead to the loss of oceans over geological timescales, evaporating Rivers lakes, oceans, and eventually aquifers and hydrated rocks as they slowly replenish the oceans, although one consequence of this can be a reduction in the surface of water on the planet and a reduction in humidity, possibly leading to stabilization of a post-greenhouse lake and desert world, 9r even cyclic behavior where the ocean refills and boils and refills and boils. Regardless, large oceans won't happen on planets that have exceeded this limit.

  2. What normally keeps planets from reaching these limits? The answer is the carbon cycle. Rock weathering, especially of Calcium rich rocks, allows carbon dioxide to be removed from the atmosphere at a rate which is exponential with temperature. These then evaporate, enter the ocean, and get recycled into the crust due to tectonics. This is why Earth's CO2 levels have changed inverse to a brightening sun. Any world with land, water, and CO2 as the principle atmospherically stable greenhouse gas is going to behave similarly.

If it gets a little bit colder than the recent ice age glacial periods, ice develops a powerful feedback loop. Once this reaches the tropical band, the entire world can be frozen within just decades, plunging the temperature to extremely low levels and allowing ice to build up in the oceans and on the surface. Technically, the planet will still have a habitable ocean, but it is largely cut off from sunlight, and there is precious little evaporation as the surface becomes an ice ball. Over geological timescales, copious amounts of CO2 can correct this snowball state and immediately cause a hothouse, with all of this ice rapidly melting even at the poles, but it could be tens of millions of years with well over 99% of the land and water covered in ice the entire time. Volcanic material will eventually discolor the ice as well.

However if it is too cold, CO2 builds up to insane levels. It can begin raining dry ice or liquid CO2, or forming clouds of the stuff that overwhelm the greenhouse effect and plunge a planet into deep cold, permanently stabilizing the snowball state. The oceans will likely remain but they will be almost entirely covered in ice and could eventually even freeze solid. This outer limit seems to be something like 1.6 AU for the sun, about 37% the brightness present at Earth. Mars is definitely in this zone but Mars is too small for plate tectonics to last.

Do note however that the bluer the light incident on a planet's surface, the brighter the light the planet needs to keep surface oceans. It is estimated that if Earth were in orbit of a 3000 K red dwarf, the carbon-silicate cycle would already have failed and subjected the planet to at least moist if not runaway greenhouse effect. Similarly, Mars would be beyond the outer edge of the HZ if it were in orbit of a blue star, although in reality blue stars won't last long enough for this to matter for habitability.

I guess the point is, if you want surface oceans that are stable over geological time, you probably need to keep it between about 5-45 °C. If you want a world that's in the process of losing its water but not actually in a runaway state, I think there are probably ways to get somewhat past 70 °C but not for long. At no point can surface oceans be supported below the vapor pressure of water at the planetary surface temperature, so for example water on Venus isn't viable below 50 km or so, and thus, there can be no surface water.

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